We have found a high frequency (1-2 percent) of non-overlapping antigenbinding cells for each of two antigens, beta-galactosidase and horseradish peroxidase. About 1 percent of beta-galactosidase (BGZ)- binding cells also can bind horseradish peroxidase (HRP), suggesting a random assortment of specificities on multipotential cells. Our major objectives are discovering how these multispecific precursor cells arise and differentiate to unispecific producing cells. We will purify BGZ-binding cells by binding to a pseudo-substrate affinity column and eluting with excess substance, free BGZ, or a variety of other tactics. Purified cells from unimmunized animals will be compared to those from primed animals to determine whether specificity restriction has occurred. By affinity fractionation of binding T and B cells, followed by functional assay, we will try to examine the relevance of antigen-binding to subsequent immune responsiveness. Several questions about the nature of receptors will be asked: their origin in ontogeny and the frequency of double binding fetal cells; receptor biosynthesis - are they all synthesized by the cell bearing them?; how many receptors are there on T and B cells?; how does antigen affect their surface distribution? The T-function of thymus antigen-binding cells will be studied upon adoptive transfer and in helper experiments. The genetic and clonal basis of antigen-binding in T and B cell lines will also be investigated.